With each heartbeat, a small amount of calcium entering the cell through membrane channels triggers the release of a larger amount of calcium from intracellular stores. The resulting increase in intracellular calcium has the dual effects of both: 1) enabling contraction and 2) influencing the ionic currents that shape of the action potential. Improper transport of calcium in cardiac myocytes can therefore contribute to both impaired ventricular function and lethal cardiac arrhythmias in disease states such as heart failure. Studies suggest that unstable calcium regulation and arrhythmias are associated with increased leak from intracellular stores to the cytosol. In particular, the inherited disorder catecholaminergic polymorphic ventricular tachycardia (CPVT) results in both increased leak and dangerous ventricular arrhythmias triggered by spontaneous release of calcium. This disease therefore represents an obvious example of the close links between ion transport and electrical signaling in heart cells. However, the mechanisms by which calcium link can increase arrhythmia risk remain unclear. We hypothesize that, because of the inherent complexity of calcium signaling and competing effects within cardiac myocytes, leak is only deleterious under certain conditions. A combination of innovative experiments and computational modeling will quantitatively determine the factors that control calcium leak and define the boundaries of when leak is dangerous and when it is protective. Together the studies proposed will yield significant insight into the factors that influence arrhythmia risk in CPVT and in heart failure. The project can be sub-divided into the following Specific Aims: Aim 1: Determine, in healthy cells, the factors that control calcium leak. Aim 2: Determine the mechanisms underlying increased leak in heart failure. Aim 3: Determine the mechanisms by which altered gating of ryanodine receptors can increase the risk of arrhythmia despite reduced sarcoplasmic reticulum calcium content. The work will provide fundamental new information concerning both the normal regulation of Ca2+ in healthy heart cells and the defects that occur in pathology. By developing a quantitative framework for understanding normal and defective calcium release, these studies can help identify thoughtful targets for cardiac therapies